In conventional systems, computer networks typically have been viewed as being divisible into several layers. The Open Systems Interconnection (OSI) reference model was established by the International Standards Organization (ISO). The OSI reference model defines a computer network as having seven layers ranging from a physical layer to an application layer. A number of different protocols have developed for use at the respective layers of a computer network. The Asynchronous Transfer Mode (ATM) protocol is a layer 2 protocol. Layer 2 is the data link layer and is responsible for transmitting chunks of information over a data link. The Internet Protocol (IP) is an example of a layer 3 protocol. Layer 3 is the network layer, which is responsible for enabling any pair of systems in the computer network to communicate with each other.
In conventional systems, ATM networks had been viewed as separate universes from IP networks. ATM networks work well for a subset of services, and IP networks work well for a different subset of services. Given that neither IP nor ATM offer a complete multiservice solution, many service providers choose to operate dual networks. IP networks supports applications such as Internet access and virtual private networks, whereas ATM networks supports frame relay, virtual private networks, circuit emulation, private branch exchange (PBX) and other applications where reliability and quality of the service (QOS) are a priority.